Roll with hydraulically supported,yieldable cover structure



Oct. 27, 1970 R. E. JAMES 3,535,760

ROLL WITH HYDRAULICALLY SUPPORTED, YIELDABLE COVER STRUCTURE Filed Aug. 22, 1968 FIG. 2 INVENTOR RALPH E. JAMES ATTORNEY United States Patent 3,535,760 ROLL WITH HYDRAULICALLY SUPPORTED, YIELDABLE COVER STRUCTURE Ralph E. James, Brussels, Belgium, assignor to The Polymer Corporation, a corporation of Pennsylvania Filed Aug. 22, I968, Ser. No. 754,532 Int. Cl. 1321b 31/32; D06c 61/00 U.S. Cl. 29-113 7 Claims ABSTRACT OF THE DISCLOSURE SPECIFICATION This invention relates to a covered roll having a hydraulically supported soft cover rotatably positioned over a mandrel. Specifically, it relates to a fluid-filled longitudinal chamber in the mandrel into which the roll cover can deflect.

THE BACKGROUND AND THE PROBLEM Covered or filled rolls for the final surface smoothing of textiles and papers are widely used in conjunction with hard metal rolls as in a supercalender. The function of the combination of a soft and a hard roll in a supercalender is to provide a nip of a significant width between the rolls, generally ranging from A to inch in width. While it is considered important thatthe nip provide some finite dwell time during which the web is held in compression in the nip, it is also generally believed necessary that there be relative movement of the surfaces of the hard and soft rolls. This relative surface motion provides a smoothing or ironing action which is believed to be necessary to the attainment of a satisfactory surface gloss on the web. Such relative surface movement occurs when a soft roll cover is deformed so as to conform to the reverse curvature of the mating metal roll. Inasmuch as the material of the roll cover is subjected to compression between the relatively unyielding surfaces of the mandrel of the filled roll and mating hard metal roll, the nip results from a momentary reduction of the thickness of the wall of the soft roll cover. Such action causes alternate elongation and contraction of the surface of the soft roll cover thus providing for the relative surface movement and the desired smoothing action on the paper. A roll cover having a compressive modulus of about 100,000 to 600,000 p.s.i. is considered soft and can be used in this type of operation.

For many years rolls comprising a metal shaft (mandrel) and a covering of a softer material such as com pressed cotton or cellulosic materials have been used in various calendering machines. In such devices webs are passed between the nips of one or more pairs of rolls, generally with a covered roll working in cooperation with a steel or chilled iron roll. Such machines work well insofar as finish of the web is concerned. However, these soft rolls are very easily damaged, for example by tears or folds of paper passing through the nip and must be removed frequently for refinishing. Refinishing is expensive per se, and, additionally, causes so much interruption in the operation of supercalenders that it is not practical to use them in line with papermaking machinery. Even though the speed of webs through calenders is of the Patented Oct. 27, 1970 same order as the speed of the basic manufacturing machinery, it is necessary to have two calenders for each basic machine because of downtime caused by roll damage.

In recent years there has been limited use of plastic roll covers for such smoothing operations since it has been demonstrated that damage resistance is sufficiently great to permit the use of supercalenders in line with papermaking machines. In general, these covers have been made of nylon, polycarbonates, and other highstrength plastics which are much more resistant to damage than are the traditional rolls. These materials have been shown to be quite capable of withstanding the action imposed by supercalendering so long as the physical forces are relatively uniform across the width of the roll.

However, complete uniformity does not exist, and when localized overloads occur, the natural heating effect resulting from mechanical inefiiciencies within the body of the plastic causes an increase in the wall thickness of the cover at that point. Since, in conventional calendering equipment, this extra wall thickness cannot escape being compressed to the same thickness as the rest of the wall adjacent to the area of overload, an even greater compressive force occurs at exactly the point of maximum initial overload. If the rate of heat generation exceeds the rate at which heat can be removed from the cover at the critical spot, an unstable condition results, the temperature rises at an increasing rate, and subsurface melting occurs. Thus if plastic roll covers are used in machines of the conventional design, they are subject to a type of failure and known as meltouts which result from local overloads and local melting of the plastic by internal hysteresis losses.

PRIOR ART SOLUTIONS This phenomenon is described rather fully in Koch US. Pat. 3,091,173, wherein it is proposed to reduce the incidence of melt-out failures by use of plastic materials having low hysteresis losses and low coefiicients of thermal expansion. While this improvement is in the right direction, it does not constitute a complete cure for the problem. It merely permits a decrease in the degree of uniformity of web and machine loading conditions that can be tolerated without failure.

Another approach to improved performance of calender rolls is described in Kuster US. Pat. 3,023,695. The basic objective of the designs described by Kuster is to provide more uniform pressure across the width of calendering rolls without the use of crowning. Such an improvement is obviously desirable for several reasons, and it also constitutes an improvement in the ability of plastic covers to operate under mill conditions without melting. However, the hydraulic loading methods described by Kuster do not completely eliminate the cause of nonuniform loading of calender nips. The heavy steel shell used in these rolls is very close to the cover and is too stiff itself to yield under one type of irregularity in web thickness that is frequently encountered in paper and cloth manufacture. I'refer to a relatively narrow ridge running lengthwise in the web which causes a high pressure area only a few inches wide. When such conditions exist, the Kusters roll is unable to relieve the overload because of the stiffness of the steel shell. While it' is true that the Kusters roll permits gradual changes in web thickness or hardness to be reflected in changes in the geometry of the roll shell, this action cannot occur in response to the narrow ridges in the order of a few inches wide, that are frequently encountered.

OBJECTS OF THE INVENTION It is the object of this invention to provide equipment and methods for the calendering of continuous webs which will permit the use of high hysteresis loss plastic roll covers without danger of melting of the plastic despite the type of uneven loading described above.

It is a further object to provide the means for obtaining more uniform finishing of webs, especially paper webs, even though uneven thickness exists.

It is a further object to permit the use of reasonably wide tolerances in the uniformity of wall thickness in the plastic covers used in the practice of the present invention.

These and other objects will become clearer as the invention is described more fully below.

SUMMARY OF THE INVENTION The present invention results from recognition of the crucial role played by the immutability, relative to the cover wall, of the two metal surfaces between which it is compressed. The novelty of the present invention lies in a hydraulically supported roll cover and preferably in a longitudinal fluid filled chamber at the nip which enables the roll cover to escape the absoute compression coerced upon it by conventional calendars, but at the same time provides a nip of normal width and permits the relative movement of, the outer surface of the roll cover against the mating roll as it passes through the nip.

THE DRAWINGS The invention is most easily understood by reference to the presently preferred embodiments shown in FIGS. 1 and 2. FIG. 1 is an end-view in cross-section of a covered roll taken through lines II of FIG. 2. FIG. 2 is a side view in cross-section of the covered roll of FIG. 1 taken through lines IIII in FIG. 1. The arrangement shown comprises one of several possible embodiments of the principles of the present invention.

In FIGS. 1 and 2 a soft roll 1 is mounted on a stationary mandrel 2 sutficiently loosely so that the cover is rotable on the mandrel to form the roll of this invention. This roll is mounted between hard rolls 3 and 4 in a su per-calendar. At the top and bottom of the mandrel are longitudinal chambers 5, which run the entire length of the mandrel except for a few inches at the ends. These chambers are connected bychannels 6 to a conduit 7. To the left and right of the chambers and likewise running most of the length of the mandrel are two longitudinal side grooves 8 which are connected by channels 9 to another conduit 10. Shown in FIG. 2 are two circumferential grooves 11 which are located at each end of the roll and are connected by channels 12 to conduit 10. Conduits 7 and pass out of the roll to pumps which provide and control the fluid pressure.

In the presently preferred embodiment shown in the drawings, longitudinal chamber 5 acts as a partial substitute for a solid mandrel in that it is filled with a fluid under pressure, and hence, resists deformation of the roll cover in a manner similar to, but not identical to, the resistance that would be offered by the mandrel itself had it not been cut away as shown. The crucial difference in performance results from the tendency of the hydraulic fluid to yield to any differences in compressive force exerted upon it. The important differences, from the standpoint of this invention, are that the fluid will conform to both differences in web thickness and to differences in the wall thickness of the roll cover. Thus, local overloads resulting from excessive paper thickness, from local overheating of the cover, or from variations in the thickness of the wall resulting from manufacture of the cover or as a result of uneven wear of the cover, can be relieved by corresponding deformation of the inside diameter of the cover against uniform hydraulic pressure rather than a mechanical support. This result can best be achieved if the width of chamber 5 is about /2 to 3 times and preferably l to 2 times the width of the nip which is usually about to /1 inch. The depth of chamber 5 does not appear to be critical although it should be deep enough to relieve any deformationsof the cover which may occur.

Means other than a longitudinal chamber may also be used to hydraulically support the roll cover.

The purpose of side grooves 8 is to relieve pressure resulting from the Reynolds effect and to provide for a positive film of oil between the moving inner surface of the cover and the stationary portions of the mandrel adjacent to the main chamber. Since this interface must support the difference between the compressive force exerted by the calender stack upon the nip and the opposed force of the hydraulic pressure acting upon the ID of the cover, and since the relative motion of these two surfaces is as great, virtually, as the speed of the web, it is essential that hydrodynamic lubrication be maintained between these surfaces.

These side grooves represent a preferred embodiment and may not be required in all cases. For example, using the same hydraulic longitudinal chamber, but without the side grooves, it is apparent that the use of a gradual wedge type entry into the unmodified portion of the roll mandrel will tend to permit hydrodynamic lubrication of the sort usually found in plain bearings. Thus, without any side grooves, the system will perform satisfactorily as a result of the Reynolds effect, and the support of the rotating plastic roll cover on a film of oil which is dragged into the space between the mandrel and the cover by the relative movement of the two surfaces. Inasmuch as it is anticipated that the roll described in this and the preceding example will normally operate with two pressure nips, and hence with two hydraulic chambers, it is apparent that the Reynolds effect need extend only for /z of a roll diameter rather than for the entire diameter. Two chambers are preferably used for each longitudinal chamher 5, although fewer or more may be used if desired. They are generally located about 1 or 2 inches from the main chamber and are made large enough to provide or take up the required amount of oil.

The circumferential grooves 11, at the end of the roll act in a similar manner, and serve both to limit frictional contact between cover and roll, but also to help prevent leakeage of oil from the ends of the roll. They are connected to the same conduit as the side grooves by means of a number of channels. These radial grooves are also a preferred embodiment and may not be required in all cases. Also provided are means for filling the chambers and grooves with a fluid under pressure. The system of channels shown in the drawings is one such means. The fluid used is preferably oil although other fluids may also be suitable.

Not shown, since they do not comprise a part of this invention, are the means of sealing the end of the cover. Suitable means include ball or roller bearings with conventional oil seals, which can be used to completely eliminate oil leakage from the roll ends. Roll cover seals are illustrated in Kuster Pat. 3,023,695 and elsewhere.

OPERATION OF- THE ROLL As is illustrated by the drawings, the roll of this invention is noramlly used as a soft roll between two hard metal rolls. The roll could also be one of a series of rolls used in the conventional calenders stack, where three to eleven rolls are vertically stacked, with a king and queen roll at the bottom and top, respectively, and with smaller rolls, alternatively hard and soft, between the two extreme rolls.

It is apparent that conforming deformation of the inside diameter of the cover, as well as the formation of the nip itself, result in some bending of the short beam comprising the section of cover immediately over the hydraulic chamber. The amount of beam bending which occurs is a function of the total nip pressure, of the thickness of the cover, of the width of the chamber, of the modulus of elasticity of the cover, and of the hydraulic pressure in the chamber. In practice, the formation of the nip cannot be the result of bending only, or most of the desired relative motion of the surfaces of the rolls in the nip will be lost. Hence, it is desirable that the combination of forces be such that a substantial portion of the nip deformation be the result of reduction in the thickness of the wall of the cover, and that beam bending occurs primarily to relieve overloads. It is apparent, therefore, that the pressure in the hydraulic chamber should be quite high-generally ranging from a minimum of about 100 p.s.i. to a maximum of about 3000 p.s.i.; however, the effect of the pressure in generating hoop stresses in the cover cannot be so great as to split the cover and should not increase the diameter by more than 0.5%. The preferred pressure is between about 200 and about 1500 p.s.1.

It is apparent that there are necessary relationships between the width of the nip, the width of the main hydraulic chamber, and the effective hydraulic pressure in the chamber. In the first place, the effective total force exerted by the hydraulic fluid against the area defined by the hydraulic chamber can be adjusted to be greater than the total force imposed by the mating metal roll on the outside diameter of the plastic cover. In such case, the oil will escape to the side grooves, and unless the pressure in the side groves is maintained at a high level, a selfregulating pressure device will result. The self-reguatling feature, of course, results from the maximum flow of oil available from the pumping source through the passages leading to the main hydraulic chamber. It is apparent that, if the width of the groove of the chamber is substantially the same as the width of the nip being used in a calender ing operation, that the total force exerted by the hydraulic oil in the chamber must approximately balance the force being exerted by the calender stack itself. Otherwise, the roll cover would be forced into high pressure contact at the edges of the hydraulic chamber, thus causing damage and possible melting of the inside diameter of U the cover. If, however, the hydraulic pressure, multiplied by the effective area of the chamber, balances the force exerted by the roll mating with the plastic cover, then the effective force at the edges of the chamber will be zero, and there will be no such tendency. If the hydraulic oil pressure is regulated to be slightly higher than the effective load imposed, then leakage will occur to the side chambers, and the self-regulating feature will come into play. It is possible to use a main hydraulic chamber either narrower or wider than the effective nip. If the chamber is half the width of the effective nip, then the hydraulic pressure will have to be high enough to support the entire imposed load, and compression of the roll cover will occur in a manner quite similar to what would occur if the mandrel were solid, with the notable exception that abnormally thick sections of roll cover would deform into the space provided by the main hydraulic chamber, thus avoiding a local over-compression area.

It is possible to provide a control device in addition to those mentioned above by maintaining the oil in the side grooves under a positive pressure. Thus, escape of oil from the roll system through the side chambers would be regulated by a pressure control relief valve, so that no oil would escape from these grooves unless the pressure exceeded the pre-set value. This adjustment of the maximum pressure in the side chamber system could be used to assist in precluding contact between the roll cover and the mandrel.

It is apparent that other design means could be employed to accomplish the same result-that is, to permit the system to accommodate excessive wall thicknesses of a localized sort in the plastic roll cover, and to permit such excessive Wall thickness to escape being absolutely compressed and thus to become even hotter until melting occurs.

I claim:

1. A roll comprising a soft cover rotatably mounted on a mandrel wherein said mandrel is provided with (A) a longitudinal chamber at its periphery;

(B) means for maintaining a fluid in said chamber under pressure;

(C) at least one longitudinal side groove at its periphcry; and

(D) means for maintaining a fluid in said side groove under pressure.

2. The roll of claim 1 wherein said first and second means are two systems of channels and conduits.

3. The roll of claim 1 wherein said mandrel is also provided with a circumferential end groove at each end and means for maintaining a fluid in said end grooves under pressure.

4. The roll of claim 1 wherein said fluid is oil.

5. A roll, the peripheral surface of which is adapted to be placed in lengthwise pressure contact with a substantially unyielding surface of a cooperating pressure member whereby a nip is formed between the roll and the member, said roll comprising a fixed mandrel, a cover formed of a yieldable material mounted for rotation on said mandrel, a chamber on the periphery of the mandrel extending lengthwise thereof and adapted to be located in alignment with the nip, said chamber having a width of up to about three times the width of the nip and providing a space into which said cover can deflect in response to localized compression overloads at the nip, means including a system of fluid passageways for maintaining a fluid under pressure in said chamber, and pressure regulating means in communication with said chamber for relieving the fluid pressure in the chamber upon deflection of the cover into the chamber.

6. A roll according to claim 5., wherein said pressure regulating means comprises at least one channel in the periphery of the mandrel, said channel running lengthwise of the mandrel and being offset from the chamber in the direction of rotation of the cover, the cover and the mandrel being so toleranced that the cover is supported by a cushion of fluid under pressure between the chamber and the channel.

7. A roll according to claim 5, wherein said cover is formed of an organic plastic material.

References Cited UNITED STATES PATENTS 634,584 10/1899 OConnor 38--52 3,196,520 7/1965 Appenzeller 29116 3,254,515 6/1966 Schiffer -170 x 3,386,148 6/1968 Robertson 29--116 FOREIGN PATENTS 625,759 9/1961 Italy.

6,509,484 2/1966 Netherlands.

1,277,793 9/1968 Germany.

WALTER A. SCHEEL, Primary Examiner L. G. MACHLIN, Assistant Examiner 

